How do the boys in Lord Of The Flies sustain themselves in Chapters 1, 2, 3, and 4?
Chapter 3 begins with a description of Jack hunting. He isn't able to get anything but afterward, in talking to Ralph, he reveals his deep desire to get some meat.
This question seems to apply to food as sustenance. But the boys are "sustained" in other ways. Jack is sustained (kept motivated and occupied) by a growing need to hunt and potentially kill. Ralph and Simon are sustained by their work on the shelters and their hope of being rescued. This notion of being sustained implies a notion of what keeps the boys motivated and/or occupied.
Getting back to the question of food as sustenance, when Jack returns and is talking to Ralph, he gets some water from a coconut shell. The water comes from the fresh stream.
Later in this chapter, Simon is off walking by himself. He walks through acres of fruit trees where (as the narrator puts it) even the "least energetic" person could find plenty of food. So, here is a plentiful source of sustenance. The littluns catch up with Simon but they can not reach the fruit, so he grabs it for them.
In Chapter 4, Jack successfully kills a pig. Although Ralph is angry at Jack for having neglected the fire, he gladly eats some meat:
Ralph’s dribbled. He meant to refuse meat, but his past diet of fruit and nuts, with an odd crab or fish, gave him too little resistance. He accepted a piece of half-raw meat and gnawed it like a wolf.
So, prior to this successful hunt, Ralph (and the others) had been surviving on fruit, nuts, and the occasional crab or fish.
In Chapter 1, the boys are still figuring things out. They chase a piglet but are unable to kill it. The chapter ends with a description of the boys searching for, and devouring, any food they can find. There is no specific indication of what this food is. So, we might assume that it could be berries and other edible plants.
The boys get fresh water from the stream and they store it in coconut shells, although this source of fresh water is not specifically revealed (to the reader) until Chapter 6.
In Chapter 2, the boys are primarily concerned with getting the fire started and making smoke. It isn't until Chapter 4 that the boys are able to get something other than plant life. In this chapter, Jack and his hunters are able to kill a pig. This is well and good for having some food, but it comes at the expense of the fire. While Jack and his crew are out hunting, instead of maintaining the fire, the fire goes out. A ship happens to pass during this time. With no smoke signal from the fire, the ship has no chance of seeing them. When Ralph and Piggy admonish Jack, Jack is indignant, crying "I got you meat!"